One of the UK's busiest shopping precincts has been reopened by police after the fatal stabbing of a teenager during the height of the Boxing Day sales.

The 18-year-old victim from south London, who has not been formally identified, was killed outside a branch of the shoe shop Foot Locker, opposite Bond Street underground station, in broad daylight as horrified shoppers looked on.

Police were called at about 1.45pm, and the victim was pronounced dead at the scene. Police said a fight had broken out after youths began arguing in the shop.

Eleven people remain in custody, and police said it was too early to comment on a motive for the stabbing. A forensics team erected a tent in front of the shop to begin their examination of the scene.

Officers said the victim's next of kin had been informed. Thousands of shoppers were in the area for the Boxing Day sales, one of the busiest shopping days of the year.

An employee at the nearby Disney Store said the shop had been forced to close, adding: "We are being quite badly affected – we are unlikely to open again today." A police cordon stretched west along Oxford Street towards Selfridges.

A spokeswoman for the New West End Company, which represents 600 retailers in the West End, declined to comment on the incident or its effect on retailers. Traders in the West End had reported £15m of business in the first three hours of the Boxing Day sales.

The senior investigating officer, Detective Chief Inspector Mark Dunne, from the Metropolitan police's homicide command, said: "A number of weapons have been recovered from the scene. Whether I've got the murder weapon I don't know."

Because the area was so busy, he said, "we have probably more witnesses than we would normally have and I'm very much looking forward to them coming and speaking to us".

Later in the day, another man was injured in a second stabbing, at the junction of Oxford Street and Regent Street.

When asked whether there was any connection to the first attack, Westminster police inspector Bruce Middlemiss said: "There's nothing firm, however they are a similar sort of circumstances, youths possibly from the same south London area."

The victim of the second attack was a 21-year-old who had been stabbed in the leg. His injuries were not said to be life-threatening. Police were called to the scene at 6.20pm and three men were arrested in connection with the incident.

It is not the first stabbing on Oxford Street. In May 2008, Steven Bigby, 22, of Hackney, east London, was killed in daylight outside a McDonald's restaurant. Anthony Costa, 19, of Walthamstow, east London, was jailed in December that year for manslaughter.

In August, two male teenagers were stabbed outside H&M on Oxford Street. The teenagers, aged 18 and 15, survived the afternoon attack.